A major driver of the U.S opioid crisis is limited access to effective medications for opioid use disorder (MOUD) that reduce overdose. Traditionally, jails and prisons in the U.S. do not initiate or maintain MOUD for inmates with OUD prior to their return to the community, which places them at high risk for fatal overdose. A 2018 law (?Chapter 208?) made Massachusetts (MA) the first state to mandate that five county jails deliver all FDA-approved MOUDs (extended-release naltrexone [XR-NTX] buprenorphine-naloxone [BUP-NX], and methadone). Chapter 208 establishes a 4-year pilot program to expand all FDA-approved forms of MOUD at five county jails; two more county jails in MA voluntarily joined this initiative. The law stipulates that MOUD be maintained in individuals receiving it prior to detention, and initiated prior to release among sentenced inmates where appropriate. The seven jails must also facilitate continuation of the medication in the community on release. The Massachusetts Justice Community Opioid Innovation Network proposes to partner with these seven diverse jails and community treatment providers to conduct a Type 1 hybrid effectiveness- implementation study of Chapter 208. We will: (1) Perform a longitudinal treatment outcome study among inmates with OUD who receive XR-NTX, BUP-NX, methadone, or no MOUD in jail utilizing MA?s powerful and innovative Public Health Data Warehouse, a collection of over two dozen linked state administrative data sets, to examine post-release MOUD initiation, engagement and retention, as well as fatal and non-fatal overdose and recidivism. Propensity score methods will adjust for selection effects. (2) Conduct an implementation study to understand contextual factors that facilitate and impede delivery of MOUDs in jail and community care coordination, and best practice strategies that optimize MOUD delivery in jail and coordinated care with community partners. (3) Perform a pilot randomized comparative effectiveness trial of antagonist (XR-NTX) versus agonist treatment (BUP-NX or methadone) initiated prior to release among 100 sentenced and follow- them 12-months post-release to examine outcomes that cannot be assessed in administrative data and determine procedures, acceptability, feasibility and effect sizes to support planning for a future randomized comparative effectiveness trial. (4) Calculate the cost to the correctional system of implementing MOUD in jail, and conduct an economic evaluation from state-policymaker and societal perspectives to compare the value of MOUD prior to release from jail to no MOUD among matched controls. The Mass JCOIN team, in collaboration with the MA Department of Public Health, seven MA county jails and community treatment partners, has the experience and expertise to fulfill the study aims. The Chapter 208 initiative has important implications for future policy and practice in the justice and OUD treatment systems at the local, state, and national levels. This study?s insights into Chapter 208?s implementation will inform the efficient development of future strategies to address OUDs in jail populations nationwide.